What does the preference field mean in the routing table?

The Preference field represents the routing priority in the routing table. Equipment manufacturers will specify different priorities for different routing protocols in their respective products.

Routers often support multiple routing protocols. The support of multiple routing protocols involves the selection and coordination of multiple routes. In order to solve this problem, the concept of preference is introduced in routing parameters.

Each routing protocol generally sets a fixed preference value. The smaller the preference value, the higher the priority of the route corresponding to the protocol. Under normal circumstances, the routing priority is specified as follows:

直接路由              0
OSPF 路由             10
IS-IS的level 1的路由      15
IS-IS的level 2的路由      18
NSFnet主干的SPF路由       19
缺声网关和EGP缺省        20
重定向路由             30
由route socket得到的路由   40
由网管加入的路由         50
路由器发现的路由         55
静态路由              60
Cisco IGRP的路由         80
DCN的hello的路由         90
Berkeley的RIP路由        100
点对点接口聚集的路由      110
down状态的接口路由       120
聚集的缺省路由优先级      130
OSPF的扩展路由          140
BGP的路由             170
EGP的路由             200

The concept of routing priority is that a new route with a higher priority can replace a route with a lower priority and the same destination, and vice versa. However, in specific implementation, it is possible that certain changes will be made. For example, RIP-2 under quidway2501 provides a command to change the routing priority of RIP. This command can be used to change the priority of RIP routes. When configuring static routing in quidway2501, you can also specify the priority of the route. This is done for specific applications. However, we do not encourage changing priorities.

What needs to be distinguished is the two concepts of routing cost (metric) and routing priority (preference). Metric is for the same routing protocol. For different protocols, since they represent different meanings, it is meaningless to compare the metrics of different protocols. Therefore, to make a choice between two routes with the same information of different protocols, you can only compare the routes. priority. On the contrary, preference is for different protocols. The priority of routes with the same protocol is generally the same. At this time, metric is the criterion for choosing between two routes with the same information sink.

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